考虑邻近结构阻隔影响的基坑开挖前降水引发地层变形的特性

依托天津某地铁车站基坑实测资料开展一系列数值模拟研究，考虑邻近结构阻隔影响，探讨在坑外有/无地下结构及既有地下结构与基坑不同间距条件下开挖前降水引发的围护结构及坑外土体变形特性，通过对比各工况下基坑围挡与坑外土体变形模式、最大围挡侧移与最大地面沉降发展规律、墙后地表沉陷与基坑围挡侧移的面积关系等，揭示邻近结构对开挖前抽水引发基坑变形的影响机理. 研究表明，坑外地下结构的存在对地层运动发展有一定阻隔作用，且地下结构与基坑间间距越小，这种阻隔效应越明显；地下结构对其后方地层变形具有牵引效应，导致地下结构后方出现明显沉降槽，但随着地下结构与基坑间间距的增大，牵引效应不断减弱. 阻隔、牵引效应发挥的临界值分别为1倍、2倍的目标降水深度；当地下结构与基坑间间距处于相应临界值以内时，在基坑设计中应考虑阻隔与牵引效应的影响以得到更合理的支护与施工监测方案.

Characteristics of ground deformation induced by pre-excavation dewatering considering blocking effect of adjacent structure

A series of numerical simulations were carried out on the basis of a practical dewatering test in an excavation for metro station in Tianjin, to investigate the characteristics of wall and soil movement induced by pre-excavation dewatering considering the blocking effect of adjacent structure. The effect of the spacing between the existing structure and the excavation on the ground response was revealed. By comparing the deformation modes of retaining wall and soil, the development of the maximum wall and soil deformations and the relation between the soil loss areas induced by wall deflection and soil surface settlement, the influence mechanism of the adjacent structure on the dewatering-induced foundation pit deformation was revealed. Results show that the existence of surrounding underground structure limits the development of the ground movement, and a more apparent limiting effect would appear in the case with greater spacing between the underground structure and the excavation. In the meantime, the underground structure has a pulling effect on the ground behind it, leading to obvious surface subsidence behind the underground structure, but this pulling effect weakens continually with the increase of the distance between the underground structure and the excavation. In addition, the critical values of distance between the underground structure and the excavation were obtained to estimate the blocking effect and pulling effect, which were one and two times of the target dewatering depth. When distance between the underground structure and the excavation is within the corresponding critical values, the blocking effect and pulling effect should be considered during foundation pit design to yield more reasonable support scheme.